In recent years, optical fiber sensors, also known as optrodes, have been developed to detect the presence and monitor the concentration of various analytes, including oxygen, carbon dioxide, glucose, inorganic ions, and hydrogen ions, in liquids and in gases. Such sensors are based on the rcognized phenomenon that the absorbance or luminescence of certain indicator molecules is specifically perturbed in the presence of certain analytes. The perturbation in the absorbance and/or luminescence profile can be detected by monitoring radiation that is reflected or emitted by the indicator molecule when it is in the presence of a specific analyte. The targeted analyte is generally a part of a solution containing a variety of analytes.
Duofiber optical sensors have been developed that position an analyte-sensitive molecule in the light path of a sensor tip. The indicator molecule is typically housed in a sealed chamber whose walls are permeable to the analyte. The sealed chamber is submerged in the analyte-containing solution. The sensor tip includes a pair of optical fibers. The term "duofiber" refers to the number of fibers in the sensor tip. In comparison, a "single-fiber" sensor utilizes only one fiber in the sensor tip. In a duofiber sensor, one fiber transmits electromagnetic radiation, termed measuring signal, from a signal-generating component to the indicator molecule. The other fiber transmits the reflected or emitted light, termed indicator signal, from the sensor tip to a signal-measuring component that measures the indicator signal intensity. The configuration of the optical fibers between the signal-generating component, the sensor tip, and the signal-measuring component describes the optical fiber distribution system for the sensor system.
There are two uncommon types of sensor systems: absorption and luminescent. In an absorption system, an analyte-sensitive dye is typically housed in the sealed chamber of the sensor tip. The system operates on the concept of optically detecting the change in color of the analyte-sensitive dye. This is done by measuring the intensity of the measuring signal reflected or unabsorbed at the sensor tip and comparing it to the intensity of the original measuring signal to determine the portion of the measuring signal that was absorbed by the dye at the sensor tip. Suitable analyte-sensitive indicator molecules are known in the art and are selected based upon the particular analyte substance whose detection is targeted.
There are two major types of luminescent sensor systems: phosphorescent and fluorescent. Such systems operate on the concept of measuring the luminescent intensity or lifetime of the radiation emitted by the excitation of the analyte-sensitive molecule. The sensor irradiates the indicator molecule with light at a wavelength band corresponding to a region of analyte-dependent absorbance by the indicator molecule. Luminescent emission is measured by the signal-measuring component. The ambient analyte concentration is determined by known techniques as a function of the measured luminescent emission.
The optical fiber distribution system is an integral part of each optical fiber sensor system. The distribution system is made up of optical fibers and optical connectors. The distribution system directs the measuring signal from the signal-generating component to the sensor tip and also directs the reflected or emitted indicator signal from the sensor tip to the signal-measuring component. If the sensor is using an absorption monitoring technique, the distribution system will additionally direct a portion of the measuring signal directly to the signal-measuring component. A determination of the quantity of a specific analyte is then made by comparing the intensity of the measuring signal to the intensity of the indicator signal. As noted above, if the sensor is using a luminescent monitoring technique, the analyte concentration is determined by analyzing the emitted indicator signal over a period of time.
The efficiency and reliability of a sensor system largely depends on its optical fiber distribution system. Although current optical fiber technology may not provide a one hunderd percent signal transfer at fiber connection points, the signal reduction at optical fiber connections should be ascertainable and controllable. Variability in analyte concentration measurements that may be related to the optical fiber distribution system arise from fiber loss, fiber coupling inefficiency, fiber concentration, and response to noise, either random or periodic, produced by a variety of internal and exernal sources.
In current medical applications, it is desirable that the fiber distribution system be relatively small, flexible, and highly efficient. The size requirement becomes more crucial as in situ blood gas monitoring techniques are being developed. For example, a blood gas catheter or sensor may be inserted into and left in a patient's body for a long period of time to provide continuous monitoring of specific conditions. The catheter tip includes the analyte-sensitive indicator molecule. For the patient's comfort, the catheter tip should be as small as possible. To accommodate this desirable size characteristic, a single fiber extending to the catheter tip is desirable. The remainder of the distribution system is then sized to proportion to the catheter tip fiber for maximum efficiency.
In a single-fiber sensor system, a single optical fiber carries the measuring signal to the indicator molecule, as well as carries the reflected or emitted indicator signal from the indicator molecule. One useful characteristic of a single-fiber system is that it is reducible to nearly one-half the size of the duofiber system at the sensor tip. However, a single-fiber sensor presents problems related to the small amount of light a single fiber, as well as the related distribution system, can carry, and the ability of the system to distinguish indicator signals from measuring signals that are reflected back at imperfect fiber connections. The former problem is especially prevalent in analog-based sensors. Medical sensors are generally of this type. In analog-based sensors, the intensity of the signal produced at the sensor rather than the mere existence of the signal, as in a digital system, is significant. Each change in signal intensity that is not traceable to a constant in the distribution system will be attributed to a parameter in the monitoring process. Thus, the optical fiber distribution system must be highly predictable and reliable in order to provide useful monitoring results.